The genetic dissection of a complex process such as melanocytic transformation is a daunting task. Indeed, different stages of differentiation of human melanocytic cells, such as normal melanocytes, nevi and melanomas representing different stages of disease progression, reflect distinct gene expression patterns. An in-depth knowledge of the genetic and epigenetic controls of cellular proliferation and cell cycle progression may provide critical information regarding the conversion of a normal melanocyte to its neoplastic phenotype. Over the past decade, SIRT1, a member of SIR2 family of sirtuin class III histone deacetylases, has garnered tremendous attention in neoplastic transformation and progression. A number of critical proteins, including p53 and FoxO transcription factors, have been identified as SIRT1 substrates. The roles and functions of SIRT1 in cancer have become extremely complex and are not well understood. Whereas a plethora of studies have shown a tumor promoter function of SIRT1, a number of studies have advocated its tumor suppressor role. It appears that SIRT1 plays dual functions in different tissue contexts depending on the spatial and temporal distribution and abundance of different SIRT1 downstream targets and factors that regulate SIRT1. The role and functional significance of SIRT1 in melanocyte biology, melanocytic transformation and progression is not clear. In our preliminary data, we have found that i) SIRT1 is expressed at much higher levels in melanoma cells and tissues, and ii) inhibition of SIRT1 results in a significant anti-proliferative response in melanoma cells. Further, the observed effects of SIRT1 inhibition were accompanied with transcriptional activation of p53. This is of particular interest because melanoma is one of the few cancers which rarely possess p53 mutations and this may provide an alternative route for altered p53 regulation without actual mutations. Thus, based on available literature and our exciting preliminary data, in this study we propose to test the hypothesis that SIRT1 plays a critical role in melanocytic transformation and melanoma survival via modulating its downstream regulation of p53, FoxO, and E2F1 transcription factors. The following aims are proposed: 1) to evaluate the expression profile of SIRT1 and its specific correlations with downstream regulators (viz. p53, FoxOs and E2F1 during melanomagenesis), and to determine if SIRT1 is involved in melanocytic transformation in an in vitro model; 2) to define the involvement of p53, FoxO and E2F1 transcription factors as downstream determinants of SIRT1 in melanocytic cells; and 3) to determine the therapeutic significance of SIRT1 inhibition in vivo in athymic nude mice implanted with melanoma cells. We expect that the outcome of studies proposed in this application may define the role and mechanism of SIRT1 in melanocyte biology, melanocytic transformation and melanoma progression. This may ultimately have therapeutic implications.